The present invention relates to a facsimile apparatus which comprises a V.17 MODEM (modulator-demodulator) and performs ECM (Error Correction Mode) communication.
A conventional facsimile apparatus performs ECM communication in accordance with a procedure defined by CCITT Recommendation T.30.
A procedure of ECM transmission by a G3 (Group 3 in Classification of facsimile apparatus (Recommendation T.0)) facsimile apparatus having a V.17 MODEM will be described.
First, the G3 facsimile apparatus receives a NSF (Non-Standard Facilities), a CSI (Called Subscriber Identification) and a DIS (Digital Identification Signal) by a V 21 MODEM (300 bps), and transmits a NSS (Non-Standard Set-up), a TSI (Transmitting Subscriber Identification) and a DCS (Digital Command Signal) by the V.21 MODEM. Thereafter, the facsimile transmits a long training/TCF (Training Check) by a V.17 MODEM (14,400/12,000/9,600/7,200 bps).
After the transmission of the TCF, the facsimile apparatus receives a response from the receiving apparatus by the V.21 MODEM.
If the apparatus receives a FTT (Failure to Train) as the response, it slows transmission speed of the V.17 MODEM and repeats the transmission of the NSS, TSI and DCS by the V.21 MODEM and the transmission of the long training/TCF by the V.17 MODEM.
If the apparatus receives a CFR (Confirmation to Receive) as the response, it continuously transmits a short training, a HDLC (High Level Data Link Control) flag sequence `01111110`, a HDLC image signal and three RCP's (Return to Control for Partial Page) by the V.17 MODEM at the current transmission speed, thereafter, transmits a PPS-Q (Partial Page Signal, Q=either of MPS, EOM and EOP explained below) by the V.21 MODEM. It should be noted that the facsimile apparatus transmits as the PPS-Q:
1) a PPS -NULL, PA0 2) if the image signal indicates a complete page, PA0 1) If a PPS-Q has been received, PA0 2) If a CTC has been received,
if the image signal transmitted to the receiving apparatus indicates a partial page.
a) a PPS-MPS (Multi-Page Signal), if the current page is not the last page of the original.
b) a PPS-EOM (End of Message), if the current page is the last page and the current mode is changed.
c) a PPS-EOP (End of Procedure), if the current page is the last page and the current mode is not changed.
Then, the apparatus receives a response from the receiving apparatus by the V.21 MODEM.
If the apparatus receives a PPR (Partial Page Request) as the response, it continuously transmits a short training, HDLC flag sequence, the frames required by a FIF (Facsimile Information Field) in the PPR and a RCP and retransmits the PPS-Q by the V.21 MODEM.
Further, if the apparatus receives a MCF (Message Confirmation) as the response, it performs following procedures corresponding to the PPS-Q:
1) If the PPS-Q is PPS-NULL or PPS-MPS,
it transmits a short training, a HDLC flag sequence, a HDLC image signal of a next partial page or a next page and a RCP by the V.17 MODEM. PA1 it transmits a DCN (Disconnect) by the V.21 MODEM and releases the line to terminate the communication. PA1 it returns to phase B and wait to receive a NSF, a CSI and DIS by the V.21 MODEM PA1 it transmits a PPR requiring retransmission of the error frames to the transmitting apparatus by the V.21 MODEM, and then receives a short training and the error frames from the transmitting apparatus. PA1 it transmits a MCF to the transmitting apparatus by the V.21 MODEM and performs a next procedure in accordance with the received PPS-Q,
2) If the PPS-Q is a PPS-EOP,
3) If the PPS-Q is a PPS-EOM,
If the facsimile apparatus receives a PPR as a response to the PPS-Q four times, it transmits a CTC (Continue to Correct) having transmission speed information and then receives a response from the receiving apparatus by the V.21 MODEM.
If the facsimile apparatus receives a CTR (continue to Retransmit) as the response, it changes the transmission speed of the V.17 MODEM to a value set in the CTC, retransmits the long training, the frames required by the FIF in the PPR and the RCP, and continues to perform the procedure.
If the facsimile apparatus does not receive a CTR as the response, it transmits a DCN by the V.21 MODEM and releases the line to terminate the communication.
Next, a procedure of ECM reception by the G3 facsimile apparatus having the V.17 MODEM will be described below.
When the facsimile apparatus receives a call, it transmits a CED (Called Station Identification), thereafter, transmits a NSF, a CSI and a DIS by a V.21 MODEM. The apparatus receives a NSS, a TSI and a DCS from the transmitting apparatus by the V.21 MODEM and sets transmission speed of the V.17 MODEM in accordance with transmission speed information included in the received DCS to receive a long training/TCF from the transmitting apparatus.
If the facsimile apparatus does not receive the TCF correctly, it transmits a FTT by the V.21 MODEM and rereceives a NSS, a TSI and a DCS. If the facsimile apparatus receives the TCF correctly, it transmits a CFR via the V.21 MODEM and receives such as a short training, a HDLC flag sequence and a HDLC image signal. After the apparatus receives at least one RCP frame, it quits signal reception status by the V.17 MODEM.
Next, the facsimile apparatus receives a PPS-Q or a CTC by the V.21 MODEM. The apparatus performs following procedures in accordance with a received signal.
a) if there are error frames in the received image signal,
b) if there is no error frame in the received image signal,
i) if the PPS-Q is a PPS-NULL or a PPS-MPS, it receives a short training or an image signal indicative of a partial/next page by the V.17 MODEM. PA2 ii) if the PPS-Q is a PPS-EOM, PA2 iii) if the PPS-Q is a PPS-EOP,
it returns to the phase B after T2 (6 sec.) has passed and transmits a NSF, a CSI and a DIS by the V.21 MODEM. PA3 it receives a DCN by the V.21 MODEM and releases the line to terminate the communication.
it transmits a CTR by the V.21 MODEM, sets the transmission speed of the V.17 MODEM in accordance with transmission speed information included in the received CTC and receives a long training, a HDLC image signal and the like.
In the above-described conventional ECM communication protocol, if there is an error in a short training, or if there occur many errors in image signal frames, the transmitting facsimile apparatus repeats transmission of the same block till it receives a fourth PPR. The receiving apparatus receives the same block four times, cannot receive a long training nor correctly demodulate the image signal till it transmits the fourth PPR to the transmitting apparatus.
For this reason, if many errors occur in the conventional ECM communication, the communication time period is prolonged.